When calling a subscriber in a telephone system, as is known, a ringing signal sounds which is generally generated at a central location (the exchange). For this purpose, telephone systems comprise ringing generators for generating a ringing voltage which is output after amplification by an SLIC (Subscriber Line Interface Circuit) onto the transmission line.
In this case, the ringing voltage output by the SLIC must satisfy certain requirements specified e.g. in the reference: Telcordia (Bellcore) Technical Reference TR NWT 000057 for DLC systems. In accordance with TR NWT 57,a ringing voltage of at least 40 Vrms must be present across a maximum permitted ringing load of five REN (Ringer Equivalent Number), that is to say a maximum of five telephones connected in parallel, given a maximum line length [sic] of 930 ohms. In this case, the SLIC would have to supply a ringing voltage of approximately 70 Vrms and a current of approximately 29 mA would flow. By contrast, if the load is present directly at the exchange, that is to say the transmission line is short and only has a resistance of 400 ohms, for example, it is not necessary to feed a ringing voltage of 70 Vrms into the line in order to satisfy the minimum requirement of 40 Vrms across the load. A ringing voltage of about 43 Vrms fed in by the SLIC would be sufficient in this case. If 70 Vrms are nevertheless fed in, a current of about 47 mA flows. Thus, approximately 60% more current flows than would be necessary for the five REN, with correspondingly more power being consumed.
In battery-fed applications or applications which must still maintain the telephone service during a mains voltage failure (life support systems) and utilize batteries for this purpose, such as, for example, ISDN NTs (network terminations) or DLC (digital loop carrier) systems, a high power loss is disadvantageous.
It is known to use external ringing generators, i.e. ringing generators arranged outside a CODEC (coder-decoder), which supply a plurality of transmission lines with a ringing signal and switch the ringing signal to a desired line via relays. In order to keep the current as small as possible with short transmission lines, different resistors are connected downstream of the ringing generators, depending on the length of the transmission line. However, as a result of this measure, the no-load voltage is considerably increased and the power loss rises accordingly.